Field of Invention
The present invention relates to an actuator including a dielectric elastomer.
Description of the Related Art
In recent years, development of an actuator including a dielectric elastomer in a form of a film and electrode films bonded to both surfaces of the dielectric elastomer in a thickness direction thereof has been promoted (e.g. Japanese Patent Laid-Open No. 2009-21328 and Japanese Patent Laid-Open No. 2003-174205).
In this kind of the actuator, in the case that a voltage is applied to the dielectric elastomer through the electrode film, the dielectric elastomer is compressed in the thickness direction by Maxwell stress, and the dielectric elastomer is expanded in a direction along a surface (a direction orthogonal to the thickness direction).
Then, a thickness of the dielectric elastomer is changed by changing a voltage applied to the electric elastomer, and the electric elastomer can be expanded and contracted in the direction along the surface consequently.
In a conventional actuator shown in Japanese Patent Laid-Open No. 2009-21328 and Japanese Patent Laid-Open No. 2003-174205, in order to excellently expand and contact the dielectric elastomer in the direction along the surface, the electrode films to be bonded to both surfaces of the dielectric elastomer in the thickness direction are turned to expandable and contractable soft ones (that is, the ones with low stiffness).
However, in the actuator using such electrode films with low stiffness, the following inconvenience tends to occur.
That is, in the electric elastomer, since the Maxwell stress generated by application of a voltage becomes a size inversely proportional to a square of the thickness of the dielectric elastomer, when the thickness of the dielectric elastomer becomes thin to some extent, compression force of the dielectric elastomer diverges and increases accompanying the increase of the applied voltage, and thus the thickness of the dielectric elastomer suddenly declines. Consequently, even though the stiffness of the dielectric elastomer increases by compression, the thickness easily declines to the thickness to cause insulation breakdown of the dielectric elastomer by slight fluctuation of the applied voltage.
In particular, as seen in Japanese Patent Laid-Open No. 2003-174205, in the one to keep the dielectric elastomer in an initial state of not applying voltage in the state of being strained in the direction along the surface (a so-called pre-strain state), since the thickness of the dielectric elastomer in the initial state is relatively thin, decline of the thickness of the electric elastomer tends to drastically progress with slight increase of the applied voltage. Consequently, the insulation breakdown tends to occur.
On the other hand, in the case that the stiffness of the electrode film is increased for example in order to dissolve the inconvenience, the expansion and contraction of the dielectric elastomer in the direction along the surface is constrained by the electrode film Therefore, elastic deformation of the dielectric elastomer due to application of the voltage is obstructed. Consequently, a variable width of the dielectric elastomer in the thickness direction, or the variable width of a length in the direction along the surface becomes small, and it becomes difficult to secure a required performance of the actuator.